[silverstreak]

Any ideas that will help my mechanic quickly find the cause of this misfire. Started a few weeks ago that at about 9,000 rpm bike would seem to cut on to one cylinder. Now when cold it will cut out at 5,000 RPM, when warm if I run for about 15 seconds at 6,000 RPM and twist the throttle a little more seems to drop a cylinder but then if you give it full throttle it will mostly accelerate normally till it gets to about 9,000 then it cuts out again. I was thinking blocked fuel filter after searching this site but why would it accelerate normally sometimes on full throttle, at other times full throttle makes no difference. If accelerating full throttle from idle it will sometimes go to the rev limiter without missing at all. It also has a very spasmodic miss at idle, at some red light it will cough 3 times the next time it wont miss at all. The bike is a k8 650 with 50,000 kms on it. Valve clearances recently checked as I was worried the sound I was hearing initially at 9,000 RPM was a valve not closing fully, one exhaust valve was tight and needed re shimming, all 4 spark plugs and air filter replaced at the same time, I will not be working on the bike, thanks for you ideas. One guy on this site had a very similar problem but the answer wasn’t posted. I sent a PM to him but his posts are 2 years old, thanks for your thoughts.

 

[PTRider]

Start with the easy stuff. Are the spark plug cables pushed down tightly? Is a cable chafed? Front plug well drain hole open? Take a good look and wiggle things hunting for something that isn't as it should be.

 

[Scott E. Bonds]

Cutting out at high rpm is classic sign of partially blocked fuel filter. This is what my Vee did. I suspect you may have other issues also, but would start with flow test to check filter.

 

[silverstreak]

The mechanics thought on a blocked filter was that both cylinders would cut out when the pressure dropped too low to feed the injectors.

 

[Scott E. Bonds]

I thought the same thing and I too was a shop mechanic years ago. I can not explain why but mine felt like it was dropping only 1 cylinder also.

Another thing mine did was when it dropped the cylinder, I could slowly let of throttle and it would come right back. I could cycle back and forth doing this and get it to rev higher.

Mine just felt like a bike with a carb running too lean.

 

[silverstreak]

Fuel pump and filter ok, he thinks a coil, testing tomorrow Have to take the car to work tomorrow, hope it rains.

 

[RichDesmond]

Fuel pump and filter ok, he thinks a coil, testing tomorrow Have to take the car to work tomorrow, hope it rains.

Might be spark plug caps on the front cylinder. Not uncommon on SVs, the front cap loosens up over time and don't make good contact. Replacing the caps fixes it.

 

[Boit4852]

What comes to mind and is an easy check is a corroded coil ground...or any other ground in the ignition system....or electrical system for that matter. Statr from the spark plug and work your way back. Not just visually look at things, but unscrew, unplug, and then re-connect, re-screw everything. A bike that has been washed often will cause electrical connection corrosion. By the way, is it known which cylinder is cutting out? Did you pull both plugs for comparison?

 

[silverstreak]

Not doing the work myself but trust the mechanic, so far he has ruled out fuel filter, coil, plugs, leads, ecu, throttle linkage rose connections (suzuki's suggestion along with tps setting) might have to end up trading it in, when does that new wee come to Aus? It's sure not helping my argument to the minister for finance about how economical it is to run a bike.